In the production of a cured epoxy resin product in film or sheet, an epoxy resin comprising a high-molecular-weight epoxy resin as an essential ingredient, typically a phenoxy resin, has been used. Phenoxy resins whose main skeleton is constituted of bisphenol A or bisphenol F have been used widely, but they still have problems in heat resistance, thermal expansion property, and thermal conductivity. For example, JP7-202418 A describes a copper foil coated with an adhesive based on a high-molecular-weight epoxy resin of bisphenol A type and multilayer printed wiring boards fabricated therefrom were inferior to the conventional multiplayer printed wiring boards in heat resistance and thermal expansion property.    Patent document 1: JP7-202418 A    Patent document 2: JP2-45575 A
A phenoxy resin prepared from bisphenol A and bisphenol S (that is, 4,4′-dihydroxydiphenyl sulfone) is disclosed in JP2-45575 A as an example of a phenoxy resin showing improved heat resistance. However, this phenoxy resin is higher in viscosity and harder to manipulate as resin than phenoxy resins of a comparable molecular weight and is still not sufficient in heat resistance; besides, it is virtually insoluble in common solvents such as toluene and methyl ethyl ketone, which is a defect as far as handling is concerned. Moreover, the phenoxy resins known thus far are mostly amorphous solid and their properties deteriorate sharply above their glass transition temperatures. Still more, the paint industry has demanded for phenoxy resins further improved in gas barrier property.